Alexis told Rhode Island he was “hearing voices” in August

Aaron Alexis, 34, is the contractor the FBI has identified as a suspect in the mass shooting at the Washington Navy Yard Sept. 16, 2013.

Aaron Alexis — the man authorities say is responsible for killing 12 people at the Washington Navy Yard — told Newport, Rhode Island, police last month that an individual “had sent three people to follow him and to talk, keep him awake and send vibrations into his body,” according to a police report.

According to that report, which is related to an investigation into a harassment complaint at a Marriott hotel in Newport, Alexis said he first heard the people “talking to him through a wall” at a Residence Inn in Middletown, Rhode Island, where he’d been staying. He packed up and went to an unidentified hotel on a Navy base in Newport where he heard the same voices talking to him.

He moved to a third hotel, the Marriott, according to the police report. There, Alexis first told authorities that the three individuals spoke to him through the floor and then the ceiling. Alexis said the individuals were using “some sort of microwave machine” that sent “vibrations through the ceiling, penetrating his body so he cannot fall asleep.”

He told authorities, according to the police report, that “he does not have a history of mental illness in his family and that he never had any sort of mental episode.”

As investigators scoured military contractor Aaron Alexis’ past for clues to what sparked his shooting rampage at Washington’s Navy Yard, surviving victims healed and the nation paused to mourn its latest mass shooting.

“To my knowledge, there is no explanation for the violence that occurred yesterday,” Senate Majority Leader Harry Reid said Tuesday after a moment of silence honoring the 12 people who died when Alexis opened fire inside Building 197 of the Navy Yard.

A former sailor with a history of outbursts and what two Navy officials described as a “pattern of misconduct” in the service, Alexis had recently contacted two Veterans Affairs hospitals, possibly seeking treatment for psychological issues, two law enforcement sources told CNN.

Friends also said he was upset about pay and benefits issues from a previous contracting stint, but said they never expected such violence from him.

But investigators have publicly offered no clues as to what his motive may have been. Authorities said they were certain he had acted alone. Terrorism hasn’t been ruled out but seems unlikely, Washington Mayor Vincent Gray told reporters.

The Navy Yard was closed to all but a handful of essential Navy employees Tuesday as authorities worked a detailed investigation of the shooting scene.

Authorities have recovered three guns from the scene — a shotgun and two handguns, federal law enforcement sources say. Autopsies of the victims are under way, Washington’s chief medical examiner’s office said.

Police Chief Cathy Lanier said Tuesday that Alexis had “multiple contacts” with officers from several agencies during the shooting spree and before he was ultimately shot to death. She said she had “no doubt” that the actions of officers saved numerous lives.

And amid questions over how Alexis was able to pull off the shooting on a secured military base, CNN was first to report Tuesday that Defense Secretary Chuck Hagel ordered a worldwide review of security measures at U.S. military bases. A senior Pentagon official revealed the the information.